The present invention relates to a motor-driven disk brake system for generating a braking force by means of a torque of a motor. More specifically, the present invention relates to a motor-driven disk brake system provided with a park braking function.
As an example of a motor-driven disk brake system, there can be mentioned a motor-driven disk brake system comprising a caliper having provided therein a piston, a motor and a rotary-linear motion conversion mechanism for enabling rotation of the motor to be converted to a linear motion and transmitted to the piston. A thrust is imparted to the piston according to rotation of a rotor of the motor, to thereby press a brake pad against a disk rotor, thus generating a braking force. Normally, in this motor-driven disk brake system, a force applied by a driver to a brake pedal, or a stroke of the brake pedal, is detected by a sensor, and rotation (an angle of rotation) of the motor is controlled based on a value detected by the sensor, to thereby obtain a desired braking force.
Recently, various attempts have been made to increase an advantage of such a motor-driven disk brake system by providing the system with a park braking function.
The motor-driven disk brake has reversibility relative to a reactive force imposed by the brake pads. Therefore, to apply park braking using such a motor-driven disk brake, it is necessary to provide a means to fix the piston.
For example, in a motor-driven caliper in which a rotary motion of the motor is converted to a linear motion, a rotor of the motor is adapted to be locked by means of a solenoid actuator (hereinafter referred to simply as “the solenoid”). To use the caliper for park braking, the rotor of the motor should be locked while maintaining the solenoid in a non-energized condition. To this end, use is made of (1) a mechanism for unlocking the rotor under normal braking by placing the solenoid in an energized condition, and for locking the rotor during park braking by placing the solenoid in a non-energized condition; or (2) a mechanism using a solenoid having a latch mechanism, in which the solenoid is temporarily energized in a direction for unlocking for normal braking, and is temporarily energized in a direction for locking for park braking.
Examples of motor-driven disk brake systems of the above-mentioned type having a park braking function are disclosed in Japanese Patent Application Public Disclosure No. 2003-42199 and Japanese Patent Application Public Disclosure No. 2003-222172.
When a locking action and an unlocking action (hereinafter frequently referred to as “locking/unlocking action”) for park braking are to be effected in a motor-driven disk brake, if the locking/unlocking action becomes defective (a failure of a locking action or a failure of an unlocking action occurs), the failure (hereinafter, frequently referred to as “locking/unlocking failure”) should be effectively detected to enable an appropriate countermeasure to be taken. For example, to effect a locking action for park braking, if a plunger of the solenoid malfunctions, it is difficult to place the park braking lock mechanism in a locked condition. To effect an unlocking action for park braking, if the plunger of the solenoid malfunctions, an unlocked condition of park braking cannot be easily obtained. It is anticipated that such a problem could be readily solved by appropriately detecting a locking/unlocking failure. Therefore, a means to appropriately detect a locking/unlocking failure has been desired.
However, in the above-mentioned prior art techniques, there is no means provided to detect a failure of locking/unlocking during park braking.